1. Field of the Invention
The present invention relates to an amplifier stage comprising a microwave tube in which an electron beam interacts with a microwave.
2. Description of the Prior Art
According to standard practice, the electron beam is accelerated by a very high cathode voltage, typically of the order of 40 to 70 kV.
Furthermore, in radar applications, this cathode voltage is applied not continuously but in the form of square-wave voltage pulses that are as square-shaped as possible. This is why this cathode voltage is also called a "modulation voltage" when it is produced not continuously but in pulses.
Once accelerated, the electrons of the beam are made to interact with the field produced by an electromagnetic microwave (the wave to be amplified) shifting in the same direction as the beam and having comparable velocity.
This is achieved by means of a delay structure used to slow down the velocity of propagation of the electromagnetic microwave until a velocity, known as a phase velocity, of the same order as that of the electron beam, is reached.
Now, the structures used are most commonly structures that have a certain dispersivity in frequency, i.e. the rate of delay that the structure enforces on the electromagnetic wave varies with the frequency of this wave.
This is especially so for the delay structures used with high power amplifiers, such as the delay structures that are formed by a series of coupled waveguides and thus constitute so-called "coupled cavity" structures along which the velocity of the microwave is considerably slowed down, the electron beam then moving along the longitudinal axis of this structure so as to cause an interaction all along the length of said structure.
By contrast, the velocity of the electrons of the beam is essentially determined by the cathode voltage alone, so that this velocity is quasi-constant and independent of the frequency.
As a result, as and when the distance from the central operating frequency increases, a growing difference in velocity appears between the microwave and the electron beam. This difference in velocity will greatly reduce the efficiency of the interaction and will therefore greatly restrict the passband of the tube.
More specifically, at the lowest frequencies of the operating band of the tube, the phase velocity is high, so that the velocity of the electrons, comparatively speaking, is low (the velocity of the electrons of the beam nevertheless remains always greater than the phase velocity, this being an indispensable condition for it to be possible for the interaction to occur). The electrons will therefore yield only a small part of their kinetic energy to the microwave, giving a reduced gain.
By contrast, at high frequency, the phase velocity will be low and, comparatively speaking, that of the electrons of the beam will be high. These electrons will therefore be able to yield a very large fraction of their kinetic energy to the microwave, thus giving a high gain.
However, if the frequency is increased excessively, the phase velocity is such that the structure goes out of the interaction range, and the gain then drops very swiftly.
It is thus seen that the amplification band of the tube is determined by the band for which the following condition is met: there should be little difference between the velocities of the electrons.
It will be noted incidentally that, if there is to be interaction, these two velocities should be close to each other but not exactly identical. If they were identical, no interaction could arise. It is this condition that shall be referred to when we use terms such as "velocities close to each other", "neighboring velocities", "essentially identical velocities" etc.
Under these conditions, to obtain a high passband, propagating lines with reduced dispersivity should be available. But then the variation in the phase velocity as a function of the frequency would be low, the coupling between the wave and the beam would also be low and, consequently, the same would be the case for the ouput power.
However, with excessively dispersive delay structures, the condition of wave/beam synchronism would met only on a narrow band, and this would therefore also be the case for the bandwidth of the tube. By contrast, since the matching on this reduced band is easier to obtain, the gain of the tube would be thereby increased.
It is thus seen that the performance characteristics of prior art stages are limited by a necessary compromise that has to be found between high output power, high gain and wide passband, these three characteristics having been mutually exclusive up till now.
One of the aims of the invention is to get rid of these limitations by proposing a microwave tube amplifier stage that has both a very wide passband and a very low variation in output power throughout this passband.
To this effect, the basic idea of the invention consists in setting up a servo-link between the velocity of the electrons and that of the propagated microwave tube in order to synchronize the velocity of the wave and that of the beam for a very wide band of frequencies.